臺灣博碩士論文加值系統

二氟乙烯與三氟乙烯的共聚物(poly(vinylidene fluoride trifluoroethylene) P(VDF-TrFE))的晶相在常溫下，具有高分子晶相罕見的壓電性質(piezoelectric property)。這個研究主要藉由系統性的探索升降溫過程晶相結構的演變，來進一步釐清curie相轉變的機制。在此共聚物的晶相中，由於氟原子均排列在分子鏈的同一側， 而電負性較低的氫原子排列在另外一側。這樣的晶相排列，造成一有方向性的偶極距，使得這個晶相具有極性。但是由於氟原子佔有體積較大，因此隨著溫度上升，氟原子之間的互斥會使得這樣的排列方式變得不穩定，而發生curie相轉變，分子鏈段可以藉由熱運動而有一定程度的扭曲與旋轉，使得氟原子可以分佈在分子鏈的兩側。由在升溫過程所獲得的x-ray繞射曲線以及吸放熱曲線，curie相轉變的過程牽涉兩個不同層面的晶格排列演變。首先伴隨著溫度上升，晶格的膨脹以及分子鏈構型(conformation)的改變會先發生，然後才伴隨著鏈段的旋轉與扭曲。這個分子鏈段的旋轉，是plastic crystal的主要特徵。伴隨著晶格的演變，我們也觀察到板晶排列規則性的改變，因此推論可能發生板晶微幅的彎折。而板晶彎折的現象，會隨著降溫而恢復。當此共聚物的板晶堆疊於聚二氟乙烯的板晶之間時，伴隨著的侷限效應(confinement effect)會使得板晶的彎折受到限制，因此有部分的板晶無法經歷curie相轉變。以上的研究成果不僅對curie相轉變的機制提出進一步的論述，並首次指出curie相轉變的發生會伴隨著板晶堆疊有序性的改變。

A systematic study was carried out to decipher the mechanism of Curie transition of piezoelectric crystals of poly(vinylidene fluoride trifluoroethylene) P(VDF-TrFE). The unique polarity of P(VDF-TrFE) crystalline phase below curie transition temperature is attributed to the lattice packing of all-trans molecular chains, which allocates all the substituted fluorine atoms on one side of molecular chains and hydrogen atoms on the other side. Therefore a net dipole moment is created across the lateral packing of molecular chains. Nevertheless, due to the mutual repulsion among fluorene atoms, this all-trans conformation is not stable, and ready to change above Curie temperature, where thermal kinetic energy is sufficient to cause segmental rotation. As being illustrated by in-situ recorded X-ray diffraction and thermal analysis, the concerned curie transition is deciphered as a two-step process different from conventional one-step solid-solid transitions. The lattice expansion and change of molecular conformation are derived to take place first and appears as a progressive process, which is followed by the thermal axial rotation of molecular stems within lattice packing. This thermal axial rotation of molecular stems is similar to what has been found in plastic crystals. Accompanied with this two-step process during heating, the occurrence of lamellar bending is inferred for elucidating the decline of stacking regularity of crystalline lamellae, which reversibly recover during subsequent cooling. However, as the crystalline lamellae of P(VDF-TrFE) are confined in between the stacking of crystalline lamellae of PVDF, lamellar bending is restricted accordingly. As a result, a certain fraction of the piezoelectric crystalline lamellae were found to survive through the Curie transition. Thus, in addition to the suggestion of a two-step process as a new concept for understanding the Curie transition, the relationship between the lamellar stacking and transition of molecular packing is unveiled as well in this research.

TABLE OF CONTENTS

ACKNOWLEDGEMENT i
ABSTRACT (CHINESE) ii
ABSTRACT iii
TABLE OF CONTENTS iv
LIST OF FIGURES vi
LIST OF TABLES xii
CHAPTER 1 INTRODUCTION 1
1.1 Preface 1
1.1.1 Characteristics of Phase Transitions 1
1.1.2 Molecular Structure and Crystal Stability 1
1.2 Research Objective and Motivation 2
CHAPTER 2 LITERATURE SURVEY 4
2.1 Structural Background of Ferroelectric Materials 4
2.1.2 Curie Temperature and Curie Transition 5
2.2 Basic properties of PVDF and its co-polymer P(VDF-TrFE) 5
2.2.1 Polyvinylidene fluoride (PVDF) 5
2.2.2 Poly(vinylidene fluoride trifluoroethylene) P(VDF-TrFE) 8
2.2.3 Ferroelectric to Paraelectric Transition in P(VDF-TrFE) 13
2.2.4 Structural Phase Transition observed by Infrared Spectroscopy 14
2.3 Epitaxial Crystallization 20
2.3.1 Epitaxial Crystallization of P(VDF-TrFE) 21
CHAPTER 3 EXPERIMENTAL SECTION 23
3.1 Materials 23
3.2 Sample preparation 23
3.3 Characterization 24
3.3.1 Differential Scanning Calorimetry (DSC) 24
3.3.2 2D X-Ray Diffractometer (2D XRD) 24
3.3.3 Transmission Electron Microscopy (TEM) 25
3.3.4 Small Angle X-Ray Scattering (SAXS) 26
CHAPTER 4 RESULTS AND DISCUSSION 27
4.1 Mechanism of Curie Transition of P(VDF-TrFE) 27
4.1.1 The influence of Curie transition on the evolution of lamellar stacking 34
4.2 Confinement Effect on the Melting of P(VDF-TrFE) Crystals in Binary Mixtures 42
4.2.1 Phase stability of binary mixtures of PVDF/P(VDF-TrFE) 43
4.2.2 The influence of Curie transition on the evolution of lamellar stacking PVDF/P(VDF-TrFE) 46
4.3 Curie Transition of Binary Mixtures P(VDF-TrFE) /HMB 50
4.3.1 Epitaxial organization of P(VDF-TrFE) on HMB 57
CHAPTER 5 CONCLUSIONS 59
REFERENCES 60
APPENDIX 66

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